1. Field of the Invention
The present invention relates to wholly aromatic polyamide fibers, a sheet comprising same and a method of producing the sheet. More particularly, the present invention relates to wholly aromatic polyamide fibers having a high heat resistance and an excellent electric insulating property under high temperature and high humidity conditions, and a sheet comprising the heat resistant and electrically insulating wholly aromatic polyamide fibers and at least one member selected from binder resins and heat resistant organic polymer fibrids, and usable as an electric insulating material, and a method of producing the wholly aromatic polyamide fibers.
2. Description of the Related Art
It is well-known that wholly aromatic polyamide fibers have excellent heat resistance and mechanical strength and thus are useful for various uses.
In the case where the wholly aromatic polyamide fibers are used as a resin-reinforcing material, for example, FRP material or parts of OA machines and devices which have to exhibit a very accurate dimensional stability, however, the dimensions of the aromatic polyimide fibers in the longitudinal direction thereof and in the transverse directions at right angles from the longitudinal direction change to a great extent in an atmosphere in which an increase and a decrease of the temperature are repeated, and the change in the dimensions due to absorption and desorption of moisture in the fibers is significant. Therefore, in a fiber-reinforced resin composite laminate plate, a strain is created in the interfaces between the reinforcing fibers and the resin, and the strain causes the laminate plate to exhibit a reduced durability to repeated load, and an interfacial separation occurs. Particularly, when used as an electric insulation material while an electric current is flowed therethrough for a long period, water is accumulated in a small amount in the interfaces between the resin and the reinforcing fibers, to promote movement of impurity ions contained in the resin or fibers, or to cause an interfacial separation due to a temperature-increase and water-evaporation. As an attempt for solving the above-mentioned problems, for example, Japanese Unexamined Patent Publication No. 5-83,096, No. 62-218,425 and No. 62-225,539 discloses a method of improving interfacial adhesion of aromatic polyamide fibers to a resin by heat treatment. However, this method is disadvantageous in that the fiber-reinforced resin material has a high content of impurity ions and a high moisture content, and thus is unsuitable for electrically insulating uses. These problems have not yet been fully solved.
The wholly aromatic polyamide fibers have a very high resistance to cutting and thus are used in various protective materials as disclosed in, for example, Japanese Unexamined Patent Publications No. 4-14,277, No. 6-280,140. However, this very high cutting resistance causes such a problem in that when a resin composite laminate plate reinforced by the wholly aromatic polyamide fibers is subjected to a piercing work with a drill, the reinforcing fibers are not sharply cut, the reinforcing fibers are divided into fine fibrils or not completely cut, and thus the resultant hole has a rough inner face. Particularly, in recent piercing work, a very small size of hole is required to be formed with a high accuracy, and thus a new piercing method using a carbon dioxide gas laser has been attempted. Therefore, a solution of the above-mentioned problem is strongly demanded in industry.
Also, when the fiber-reinforced composite resin laminate material is subjected to cutting work, the reinforcing fibers are not sharply cut and the resultant fine fibrils from the reinforcing fibers project from the cut face. This problem must be solved.